In the current period we made further use of IL-12beta2 chain transgenic mice and the transgenic construct used to create this mouse in an extensive examination of the role of STAT4 in Th1 differentiation. In these studies we noted initially that STAT4 deficient cells exhibit poor expression of the IL-12Rbeta2 chain and thus raised the possibility that the major function of STAT4 is the maintenance of IL-12Rbeta2 chain expression rather than down-stream effects on IFN-gamma transcription. To examine this possibility, we first showed that STAT4-/- CD4+ T cells expressing transgenic IL-12Rbeta2 chains are unable to undergo IL-12-induced IFN-gamma production or proliferation. This result showed that in the absence of STAT4, Th1 differentiation is not possible in spite of the presence of a competent IL-12R. In further experiments, we showed that CD4+ T cells deficient in endogenous IL-12Rbeta2 chain and infected with retroviruses expressing mutated beta2 chain cytoplasmic tyrosine sites that resulted in cells expressing a beta2 chain that was unable to support STAT4 tyrosine phosphorylation are also unable to undergo IL-12 induced IFN-gamma production or proliferation. The presence of an IL-12Rbeta2 unable to activate STAT4 (but able to provide signals for other Th1 functions) again showed that activated STAT4 is necessary for Th1 differentiation in that STAT4 acts "downstream," presumably on IFN-gamma transcription. In related studies we also used this system to define the tyrosine sites on the intracytoplasmic segment of the beta2 chain that mediate both STAT4 and STAT3 activation and thus are necessary for Th1 T cell differentiation and proliferation. We found three tyrosine sites necessary for STAT4 and STAT3, two which are overlapping. This result differs from those found in human cells where only one tyrosine is able to mediate STAT4 phosphorylation. Finally, we showed that Th1 T cell proliferation induced by IL-12 could occur in the absence of activated STAT4 albeit at a reduced level. This may be due to effects of unactivated STAT4 as no IL-12 proliferation was seen in STAT4 deficient cells.